LONDON (Reuters) - Britain is to investigate newly released official papers which suggest the government of Margaret Thatcher helped India plan a deadly attack against Sikh separatists in the Golden Temple at Amritsar in 1984.

Prime Minister David Cameron ordered the review after an opposition Labour party lawmaker asked the government to disclose whether the papers were genuine and whether Britain had any role in the attack on Sikhism's holiest shrine.

Sikh groups said they were shocked and disappointed by the idea that Britain may have been involved in the Golden Temple attack, a bloody episode which angered Sikhs around the world who accused the Indian army of desecration.

The raid also remains a blot on the record of India's dynastic ruling Congress party, which faces an uphill struggle to be re-elected in national polls due by May.

The party is widely expected to announce Rahul Gandhi, grandson of Indira Gandhi who was prime minister at the time of the attack, as its candidate for the post this week.

The opposition Bharatiya Janata Party (BJP), which some opinion polls say is the favourite to form the next government, criticised Congress over the 1984 incident.

"It wanted to invade the sacred precincts of the Golden Temple no matter even if it hurt the national interest and certainly the interests of the Sikhs," Arun Jaitley, the BJP's leader in the Rajya Sabha, said of Indira Gandhi's government in a blog posted on Tuesday.

The death toll remains disputed, with Indian authorities putting it in the hundreds and Sikh groups in the thousands.

The storming of the temple, aimed at flushing out Sikh separatists who demanded an independent homeland, triggered the assassination of Indira Gandhi. Two of her Sikh bodyguards shot her in revenge for the assault four months later.

PAPERS POINT TO THATCHER ROLE

Cameron visited Amritsar last year to express regret about another bloody incident there - a British colonial-era massacre of unarmed civilians - and has been trying to court British Sikh voters ahead of a national election in 2015.

Newly released British government papers from the time, publicised by Tom Watson, a Labour party lawmaker, suggest Margaret Thatcher, the then prime minister, responded positively to an Indian government request for advice on planning the 1984 attack and sent an officer from the elite SAS (Special Air Service) to help draw up a plan.

A spokesman for Cameron's office said on Tuesday the British prime minister had ordered an investigation as a result.

"These events led to a tragic loss of life and we understand the very legitimate concerns that these papers will raise," the spokesman said. "The Prime Minister has asked the Cabinet Secretary to look into this case urgently and establish the facts."

Jeremy Heywood, the cabinet secretary, is Britain's top civil servant. Cameron and Foreign Secretary William Hague had been unaware of the papers prior to their publication, the spokesman added.

Cameron's own spokesman said separately the investigation would be conducted as quickly as possible and would also examine whether the decision to release the official papers after a 30-year secrecy rule had lapsed had been the right one.

In India, there were calls for New Delhi to hold its own inquiry into Britain's possible role in the attack.

"This is very dangerous news," said Mohammed Adeeb, an independent member of the Lok Sabha.

"If there is an iota of truth in it, then we should look deeper into it across party lines. If outside help has been taken to kill and to commit atrocities on our own people, then it is extremely shameful."

Syed Akbaruddin, a spokesman for the Indian foreign ministry, said the government was only aware of the story from the media.

"We will take it up with our UK counterparts and seek more information," he told Reuters TV.

(Additional reporting by William James in London and Shyamantha Asokan and Frank Jack Daniel in New Delhi; Editing by Stephen Addison and Mike Collett-White)

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